Bird die-offs due to natural or explicable causes

Blunt-force trauma caused by impact with transparent, silicate-based materials is the top cause of bird mortality in North America killing an estimated 900 million a year.
The second-leading cause is feral cats, which kill about 500 million birds annually, followed by high-tension wires, which take a yearly toll of around 190 million.

Paul Schattenberg, Texas A&M University | Jan 18, 2011

Whether it's a single death or a mass die-off, experts from Texas A&M AgriLife and other agencies say almost all bird mortality in Texas and elsewhere is due to natural – or at least explicable – causes. Each year in the U.S., hundreds of millions of birds die from a variety of causes, according to Dr. Thomas Lacher, head of the wildlife and fisheries sciences department at Texas A&M University in College Station.

"The larger bird die-offs we see in Texas this time of year are not all that unusual given the kind of weather related to the season, storm fronts and mass roosts of birds, especially blackbirds," Lacher said. “Mass bird die-offs in the hundreds happen all the time, but we seldom see evidence of them."

However, Texas AgriLife Extension Service wildlife specialist Dr. Jim Gallagher, who works at the Texas AgriLife Research and Extension Center in Uvalde, is someone who has witnessed a sudden, unexpected mass bird die-off.

"Many years ago, while I was living in upstate New York, I saw dozens of geese crash to the ground when they were suddenly caught in a freezing rain," he said. "The weight of accumulated ice on them made it impossible to sustain flight."

In recent years, Texas has had its share of unusual, even “bizarre” weather, Gallagher noted, and birds are especially vulnerable to the vagaries of sudden cold, unpredictable winds, hail and lightening.

"If you've ever been on a heavy commercial aircraft that the wind suddenly moved up or down 1,500 feet or more in a matter of seconds, think what that kind of force could do to a bird weighing only ounces," he said. "In an updraft, masses of birds can also accumulate ice on their wings and bodies at higher altitudes. And in a sudden downdraft, especially one associated with something like a micro-burst, a mass of them can be tossed to the ground."

Gallagher said birds thrust thousands of feet upwards by a sudden updraft also are subjected to physical stress similar to that of a diver trying to resurface too quickly.

"Basically, dissolved gases in their blood suddenly start boiling out and they get the avian equivalent of the bends," he said. "A bird flying along at 1,000 feet and suddenly being thrust upwards to 20,000 feet will be subject to the same physical effects as a diver coming up to the surface too quickly — if the rise is rapid enough."

Disease and parasites may also be factors in some mass bird deaths, according to experts at the Texas Veterinary Medical and Diagnostic Laboratory in College Station.

Necropsies done by the lab on birds from a 60-plus bird die-off Jan. 8, 2007 in Austin revealed they were "heavily parasitized." But the unusually cold weather the night before was given as a "principal factor" in this die-off, which led to the temporary closure of several downtown streets by state health officials until the incident was dismissed as a public health threat.

"There has been evidence that a few wild-bird deaths in Texas over the past several years have been associated with West Nile virus," said Dr. Randy Moore, resident director of the diagnostic lab’s poultry laboratory in Center. "We historically have seen instances of West Nile, which is predominantly carried by mosquitoes, affecting birds here in Texas, but the number of birds is very small. And currently there is no evidence that avian influenza (bird flu) or other avian viruses have been associated with mass die-offs in wild bird species in the United States."

Moore said mortality from parasites or disease is more often associated with individual or small groups of birds and is usually a "contributing factor" in these situations as opposed to a singular cause of death.

Bird control to benefit agriculture

Another reason for some mass bird die-offs in certain areas of Texas from time to time could be bird control to benefit agriculture, said Mike Bodenchuk, state director for Texas Wildlife Services, an agency of the Texas Department of Agriculture's Animal and Plant Health Inspection Service. Bodenchuk said his agency is often called on by the state's farmers and ranchers to provide control of birds which are detrimental to their agricultural operations.

"For example, we provide control to rice farmers in East Texas and to feedlot owners in the Panhandle," he said. "We use a bait that the birds ingest and fully metabolize prior to death, which typically occurs one to three days after they consume it. The toxin is not transferred to other birds, animals or humans that may come in contact with the dead birds."

Bodenchuk said birds killed by the bait typically can be found in clusters beneath their roosting areas — the trees or phone and power lines they return to after ingesting the toxin.

"Bird control in East Texas rice fields is usually targeted at blackbirds, while control in Panhandle feedlots is mainly targeted at starlings, which are an invasive species," he said.

"In the case of starling control, it also helps native 'cavity-nesting' bird species by reducing competition for living space."

He added that these activities are "well-publicized" in advance and are coordinated with the Texas Parks and Wildlife Department and Texas Department of Agriculture, so it should be relatively easy to discover if a bird die-off in a particular area was due to a control effort.

Another 'ingestion-related' occurrence was the likely reason for a mass die-off of cedar waxwings near Lake Ray Hubbard in Dallas this past spring, according to experts with the Texas Parks and Wildlife Department. They said these fruit-fancying birds were migrating north as they stopped to partake of abundant, probably partially fermented, sweet berries on bushes near the Highway 66 bridge. The birds over-indulged and many became intoxicated. Hundreds – disorientated or bloated from gorging on the berries — had difficulty flying and were hit by cars traversing the bridge.

A similar die-off took place in early January of this year near a bridge in the Lake O' the Pines area of East Texas. Parks and wildlife experts and game wardens familiar with past occurrences said the death of several hundred American coots, or mud-hens – a mass-roosting, low-flying species – was most likely due to them being frightened by a noise or predator. The panicked birds flew into the path of numerous cars traveling on or near the 155 bridge.

Another possible factor in the recent mass bird die-off in Arkansas was given as loud noise or fireworks, noted Bodenchuk, but said that this explanation warrants further scrutiny.

"We've done roost relocation actions throughout Texas using literally tens of thousands of rounds of pyrotechnics, and I can't ever recall any birds dying from 'trauma' as a direct or indirect result of that control method. However, I suppose it is possible that a flock of daytime-flying birds might suddenly be startled at night by a loud noise, panic and then fly into a nearby building or other structure."

While recent mass bird die-offs in Texas and elsewhere have grabbed the headlines and spurred the imagination of conspiracy theorists – other "explanations" range from secret military or government testing to bio-terrorism and biblical portents — the real causes of the bird mortality are far more banal, according to experts.

So what are the main causes of bird death? The Sibley Guides website, a compendium of information on North American birds and trees, contains a "Causes of Bird Mortality[1]" chart giving estimated annual bird mortality from various causes in terms of millions of birds. It turns out that blunt-force trauma caused by impact with transparent, silicate-based materials is the top cause of bird mortality in North America. That's right; birds crashing into windows is by far the continent's single-greatest cause of bird mortality, killing an estimated 900 million a year. The second-leading cause is feral cats, which kill about 500 million birds annually, followed by high-tension wires, which take a yearly toll of around 190 million. Some lesser causes of bird mortality noted on the chart include cars, pesticides, communications towers and hunting, with hunting responsible for the fewest deaths of that group.

Data from other sources indicate U.S. bird mortality figures represent roughly half of those on the Sibley chart. Additionally, the National Audubon Society and U.S. Parks and Wildlife Department state that "loss of habitat" is truly the single-largest threat to birds, and that this factor is most responsible for the overall reduction in U.S. bird populations. However, the figures are nebulous.

"The die-offs we've been hearing about in Texas are a normal occurrence," said Lacher. "But when put together with reports of other die-offs in other states or countries that are posted in blogs and on the Internet and shown on national television, they appear to be greater and stranger than they really are."